Capacitive displacement sensor

ABSTRACT

A distance measuring system ( 1 ) is provided with a capacitive sensor ( 7   a,    7   b ), designed in the form of a differential capacitor ( 18 ), for which the partial capacitors (C 1 , C 2  have capacitances that depend on the position to be determined. The system also comprises a processing device ( 5 ) for determining the desired distance that contains, for example, a sigma/delta demodulator. The partial capacitors (C 1 , C 2 ) are triggered periodically with binary signals, wherein the trigger signals from the one partial capacitor (C 1 ) are transmitted with a phase offset to the signals from the other partial capacitor (C 2 ). The processing device ( 5 ) determines which trigger signals must be used for the evaluation. Within time windows that are synchronized with the edges of the trigger signals, a switch unit ( 22 ) allows the associated receiving signals to pass through to the processing unit ( 5 ) and blanks out all other signals. The distance measuring system ( 1 ) makes it possible to have a low number of interpolation errors with high resolution and long interpolation periods.  
     The distance measuring system ( 1 ) can be expanded by adding two essentially independent, parallel-operating relative measuring systems to form an absolute distance measuring system, which determines the absolute position solely from the current measuring values. A nearly optional enlargement of the measuring range is possible by adding further relative measuring systems.

[0001] The invention relates to a capacitive distance measuring systemand in particular a capacitive length measuring system forelectronically detecting and displaying a position or a displacementdistance.

[0002] A distance measuring system is known from EP 0 442 898 B1, whichsystem uses a capacitive sensor designed as differential capacitor, aswell as a signal/delta converter, consisting of an integrator and acomparator. The differential capacitor comprises two side-by-sidearranged transmitting electrodes and a joint receiving electrode, whichis arranged a short distance from the transmitting electrodes on ascale, so as to be displaceable relative to these transmittingelectrodes. Depending on the displacement position, the receivingelectrode overlays the individual transmitting electrodes differentlystrong, which leads to changes in the capacitances for the partialcapacitors of the differential capacitor. These changes represent ameasure for the displacement distance to be measured.

[0003] In order to determine the partial capacitance ratio, theindividual transmitting electrodes of the differential capacitor are fedduring each cycle with charge packets of a predetermined size butdifferent polarity. A positive charge packet is transmitted to the onetransmitting electrode, provided the total charge amount transmitted viathe receiving electrode to the integrator and integrated therein isnegative. A negative charge packet is transmitted to the othertransmitting electrode if the total charge is positive. The number ofrespectively transmitted packets is detected with associatedinterpolation counters. The ratio of these counters relative to eachother reflects the sought after ratio of the partial capacitances.

[0004] Several transmitting electrodes are arranged in a row for themeasuring of longer lengths. If the measuring range that is fixed by onepair of transmitting electrodes is abandoned during the displacement,then a change to the next pair occurs. The number and direction of thechange operations are also counted and are used jointly with theinterpolation counters for determining the distance traveled.

[0005] The known distance measuring system has proven itself inpractical operations. However, interference signals resulting from thetriggering of transmitting electrodes, the display and other sources ofinterference are also integrated and lead to non-linearity, reflected inthe measuring accuracy. In addition, a supply source must be providedthat transmits charge packets with a specific polarity at the selectedmoments to the corresponding partial capacitor.

[0006] Starting with this premise, it is the object of the invention tocreate a suitable distance measuring system, in particular forbattery-operated hand-held devices, which ensures the highest possiblemeasuring accuracy with the highest possible resolution. In addition,the distance measuring system should operate with a simple energysupply.

[0007] This object is solved with a distance measuring system having thefeatures described in claim 1.

[0008] The distance measuring system according to the invention isprovided with one or several capacitive sensors, which are designed asdifferential capacitors. The capacitive distance measuring can berealized extremely power saving. The capacitances used are in the pF(pico-farad) range, as a result of which the transmitted charge amountsand the resulting currents are very low. Each sensor comprises at least2, for example 16, transmitting electrodes that are preferably arrangedin a row with a constant partition. The sensor furthermore comprises oneor preferably several, for example 64, counter electrodes that are alsoarranged in a row. The electrodes are arranged on a material measure,such as a ruler or measuring tape, opposite the respective transmittingelectrodes and at a short, equal distance to these. The material measureis positioned such that it can be adjusted and in particular can bedisplaced relative to the transmitting electrodes. Thus, at least twopartial capacitors are formed in this way, wherein the capacitance of atleast one of the partial capacitors changes proportional to the distancedistance. A triggering device is used to feed a binary signal toselected transmitting electrodes for generating individual measuringsignals groups of signals at predetermined moments. The signals fortriggering the two partial capacitors are fixedly offset in phaserelative to each other, so that the resulting measuring signals do notoverlap in time if possible. If voltage pulses are used for thetriggering, the measuring signals essentially are the transmittedcurrent surges that form a charge amount (a charge packet). If currentsignals or charge packets function as trigger signals, however, voltagesare measured. A switch unit is advantageously provided, containingswitches that are closed for a predetermined interval to define timewindows with therein positioned, selected edges of the trigger signals.Inside the time windows, the associated measuring signals are allowed topass through to a processing device, which evaluates these signals usesthe results to determine the partial capacitances of the differentialcapacitor. The processing device determines the distance to be measuredfrom the partial capacitances.

[0009] The energy supply for the invention can have a very simpledesign. The trigger signals transmitted to the one partial capacitor canhave the same curve shape as the trigger signals for the other partialcapacitor. They can even have the same polarity and assume the samevalues, so that positive or negative rectangular signals, for example,are particularly suitable for use as trigger signals. Simple batterycells are sufficient for this.

[0010] It is advantageous if the phase offset and the time window arefixed such that only one selected signal edge can be observed inside onetime window. With a 90° phase offset, an equidistant spacing of onefourth of the clock cycle is obtained between the edges of the triggersignals for the two partial capacitors. The time window is smaller thanthe pulse duration of a trigger signal. It is advantageous if the timewindow essentially includes only the respective edge change, meaning itrepresents only a fraction of the total signal period. Interferencesignals consequently do not influence the evaluation during the completesignal duration, but only briefly during the edge change where theinterference distance is high. As a result, the linearity of themeasuring system is improved and a high measuring accuracy is possible.

[0011] The processing device can contain a capacitive sigma/deltaconverter that essentially consists of an integrator withdownstream-connected comparator. The integrator integrates the chargereceived from the differential capacitor. The comparator provides anoutput signal that corresponds to the mathematical sign for this chargeand determines the signal edge to be evaluated next. If both partialcapacitors are triggered with positive rectangular signals, for example,either the rising edge of the trigger signal for the one partialcapacitor or the decreasing edge, offset by half a cycle, of the triggersignal to the other partial capacitor is selected in dependence on thecomparator output. With trigger signals having different polarity,either the front or the rear edges of the trigger signals are alwaysevaluated.

[0012] The time window should be sufficiently large, so that thereceived charge packets are almost completely integrated in anintegration capacitor of the integrator, taking into consideration theinternal resistance of the source supplying the integration capacitor.The time window preferably is larger than the sum of the values for thetrigger signal rise time and ten times the time constant τ=R_(i)C,provided R_(i) characterizes the internal resistance of the voltagesource used and C the maximum possible measuring capacitance. On theother hand, the time window should be as small as possible, meaning itshould essentially only detect the signal edges and should besynchronized as accurately as possible with these edges, to limit theinfluence of non-linearity and interference signals as much as possible.

[0013] The switch unit according to one embodiment of the invention alsomakes it possible to create defined voltage conditions at the partialcapacitors of a differential capacitor. The partial capacitors are inprinciple polarized relative to each other before a trigger signal istransmitted, without this changing the integrated total charge. Therequired measuring system linearity for a high measuring accuracy isensured in this way.

[0014] The measuring system operation is synchronous, meaning thetransmitting signals and the measuring signals are controlled by thesame clock generator. As a result, interference signals are for the mostpart suppressed. The influence of low-frequency interference caused bythe activation of the display unit, for example a liquid-crystal display(LCD), or by interference signals induced on the scale can be reducedfurther if the transmitting and measuring signals are invertedperiodically after a number of clock cycles, preferably after each clockcycle. This can be realized through a periodic switch during theevaluation from the one edge of the measuring signals to the other edge.

[0015] The partition of the counter electrodes is proportional to thatof the transmitting electrode rows. The width of the counter electrodes,measured in displacement direction, advantageously corresponds to awhole-number multiple of the width of the transmitting electrodes. Thispermits large areas of overlay with correspondingly large partial ormeasuring capacitances, a simple evaluation and long rows of counterelectrodes with correspondingly large measuring ranges. In addition,each second counter electrode can be connected to ground because severalside-by-side arranged transmitting electrode pairs can be assigned to anactive counter electrode. As a result, even larger scale increments,meaning interpolation periods, and thus even larger absolute measuringdistances can be achieved. However, it is also possible to provide onlya few counter electrodes and several transmitting electrodes.

[0016] The signals transmitted to the counter electrodes are tapped viafriction contacts or non-contacting, for example via a receivingelectrode with capacitive coupling to the counter electrode. Thereceiving electrode can be arranged together with the transmittingelectrode row on a joint sensor head, for example parallel to it. Toavoid cross talk, it is advantageous if this electrode is insulatedagainst the transmitting electrodes, for example via grounded screen.Connecting lines need only extend to the sensor head and movablecontacts can be omitted.

[0017] The partial capacitances are determined through interpolation.For this, the processing unit has a counter unit with two counters,which can each have a depth of 8 to 16 bits, preferably 8, 10 or 12bits, depending on the desired resolution. One or several pulses aresent to each partial capacitor and the resulting charges are integrated.If a measuring signal is evaluated in one of the partial capacitors, theassociated counter is incremented. The counter that first reaches itsmaximum value is used as denominator for computing the interpolationvalue. The other counter determines the interpolation value.

[0018] The counters are furthermore used to determine the pair oftransmitting electrodes, which must be triggered and is sufficientlyoverlaid by the counter electrode. Few interpolation errors occur if thedegree of overlay is always within a specified range, for examplebetween 1:2 and 2:1. It is possible to detect whether this range hasbeen abandoned in that one of the counters is incremented several timessuccessively.

[0019] The transmitting electrodes can be combined into groups ofseveral, for example 6, electrodes that offer a corresponding number oftriggering options. Within the ranges fixed by the groups, the suitableelectrode pair is triggered and the counter settings determined.Stringing together transmitting electrodes can increase the scaleincrements.

[0020] The distance measuring system of one particularly advantageousembodiment of the invention is designed as absolute distance measuringsystem. That is to say a system, which supplies at any point in time thedesired absolute position, using only the currently detected values. Aconstant reading for control during the adjustment is necessary in thesame way as a resetting to zero following each start-up. The activeperiod can be reduced considerably and the measuring system can operatewith low cycle rates, thus making the design extremely power saving. Asa result, it is particularly suitable for battery-operated measuringdevices. In addition, the susceptibility to errors can be reducedfurther because the measuring system does not lose the zero point due toan excessively high traversing speed or due to contamination.

[0021] The absolute measuring system according to the invention isprovided with at least two capacitive sensors, designed as differentialcapacitors, which form two measuring systems with different partitions.In both measuring systems, the ratios of the partial capacitances of therespective differential capacitor, relative to each other, aredetermined. The absolute position can be determined from that.

[0022] A single receiving electrode only is used for one advantageous,space-saving arrangement. This receiving electrode is arranged betweenthe rows of transmitting electrodes and parallel to these in theabsolute distance measuring system. The counter electrodes are connectedelement-by-element to form a single row. Alternating in time, thetransmitting electrode rows can also function either as transmitter oras receiver for the other transmitting electrode row. This kind ofarrangement allows for the largest amount of surface area for thecapacitor plates on a given surface of the sensor head and thus a goodcapacitive coupling.

[0023] A separate processing unit can be assigned to each capacitivesensor, so that the evaluation is synchronized as much as possible. Thesigma/delta converter, however, can be used jointly by both sensors if aseparate integration capacitor that can be added is provided for eachsensor. A linking unit uses the resulting relative positions fordetermining the absolute position. However, it is also possible to haveapplications where a joint processing unit performs a serial evaluationof the measuring signals from both differential capacitors. To ensure asynchronous operation, the individual differential capacitors can beassigned buffer units for the charged, which are charged up with one orseveral charge pulses and synchronous if possible. The output voltagesfrom these buffer units can be evaluated successively and slowly.

[0024] The distance measuring system according to the invention permitsthe detection of relatively large measuring distances with a highresolution and low interpolation errors. If necessary, the measuringrange can be expanded nearly optionally. One simple option consists inreducing the width of a counter electrode row in sections and crosswiseto the displacement direction. The width of the other counter electroderow can remain the same or can even be widened in sections. In any case,specific capacity ratios between the rows of transmitting electrodes andcounter electrodes result in each section.

[0025] The switches, capacitors and operational amplifiers required forthe evaluation can be realized with standard MOS technology, which istechnically advantageous and cost-effective. The counter, comparativeand other operations can be realized advantageously in a correspondinglyprogrammed microcontroller or processor, even though a realization withcircuit engineering is possible as well.

[0026] All past embodiments related in particular to a length measuringsystem. However, the distance measuring system is also suitable for theangle measuring. For that, the transmitting electrodes can be formed onthe outside of a circular disk and the counter electrodes on the insideof a ring-shaped material measure, at a short distance to the disk andarranged such that they can rotate around their center axis.

[0027] Further advantageous details of embodiments of the inventionfollow from the dependent claims, the drawing as well as the associateddescription. The drawing shows an exemplary embodiment of the subjectmatter of the invention. Shown are in:

[0028]FIG. 1 A distance measuring system according to the invention,shown in a schematic, perspective representation.

[0029]FIG. 2 A simplified representation of the distance measuringsystem according to FIG. 1, which illustrates the individual functionalunits.

[0030]FIG. 3 A material measure for the distance measuring systemaccording to FIG. 1, in a schematic view from the top and using adifferent scale.

[0031]FIG. 4 A first arrangement of the transmitting electrodes for thedistance measuring system according to FIG. 1, in a schematic view fromthe top.

[0032]FIG. 5 A different arrangement of the transmitting electrodes in aschematic view from the top.

[0033]FIGS. 6a to 6 d Different configurations of electrodes for thedistance measuring system according to FIG. 1 and different triggeringmethods, shown in a schematic view from the top.

[0034]FIG. 7 A part of a processing unit in a sigma/delta converter,which can be used in a distance measuring system according to FIG. 1,shown as block wiring diagram.

[0035]FIG. 8 Signals for triggering the transmitting electrodes of adifferential capacitor and the output signal from the integrator of thesigma/delta converter according to FIG. 6, shown as schematic diagram.

[0036]FIG. 9 A detail of a modified processing unit that can be used inthe distance measuring system according to FIG. 1, shown as schematicwiring diagram.

[0037]FIG. 10 A section of another embodiment of a processing unit shownas schematic wiring diagram.

[0038]FIG. 11 A third measuring system for extending the measuringdistance, shown as a schematic view from the top to illustrate theprinciple.

[0039]FIG. 12 A simplified block diagram for illustrating the mode ofoperation of an absolute distance measuring system with two relativemeasuring systems.

[0040]FIG. 13 A simplified flow chart for illustrating the determinationof the desired position.

[0041]FIG. 1 schematically shows a distance measuring system 1, whichrepresents a part of a longitudinal measuring device and comprises asensor head 2 and a material measure 3, for example a ruler. Thematerial measure 3 for the present example extends through the sensorhead 2 and is positioned so as to be displaceable in direction V,relative to the sensor head. For example, the distance measuring system1 can be a sliding caliper. Sensors, which cannot be seen in FIG. 1, arearranged inside the sensor head 2 and are connected to a triggering unit4 and a processing unit 5. These units 4, 5 can be provided on or in thesensor head 2 or, if necessary, in a separate display unit 6 fordisplaying the measured length. Elements for turning the measuringsystem 1 on or off or otherwise operating it are not shown in FIG. 1.

[0042] The basic design of the distance measuring system 1 follows inparticular from FIG. 2. The distance measuring system 1 for the presentexample is an absolute distance measuring system, which determines thecurrent position either directly or solely from the momentary measuredvalues. For this, it has a first capacitive sensor 7 a and a secondcapacitive sensor 7 b, which is indicated in FIG. 2 with an equivalentcircuit diagram. The sensors 7 a, 7 b are respectively formed by a row 8a, 8 b, consisting of capacitances per unit length that function astransmitting electrodes, and associated rows 9 a, 9 b of counterelectrodes, as well as by a receiving electrode 10 a, 10 b. If only acapacitive sensor 7 a or 7 b is provided, a relative measuring system isformed, which determines the position in increments. The descriptionsapply accordingly.

[0043] The transmitting electrode rows 8 a, 8 b are preferably arrangedon a joint surface 11 of the sensor head 2, parallel to each other indisplacement direction V, as illustrated in FIG. 4. Each row 8 a, 8 bcontains several, for example 2 to 16, rectangular transmittingelectrodes 12 a, 12 b. The transmitting electrodes 12 a are narrower indirection V than the transmitting electrodes 12 b. Crosswise to this,their dimensions are preferably the same. The transmitting electrodesare connected via supply lines 13 a, 13 b to the triggering unit 4, fromwhich they receive triggering signals.

[0044] The rows 9 a, 9 b with several counter electrodes 14 a, 14 b arearranged on the side of material measure 3, which is facing the surface11 of sensor head 2. A gap or distance 15 is fixed between thetransmitting electrodes 12 a, 12 b and the counter electrodes 14 a, 14b, which is small as compared to their width. The partition ratio forrows 9 a, 9 b corresponds to that of the transmitting electrode rows 8a, 8 b. However, the width of counter electrodes 14 a, 14 b exceeds by amultiple the width of the transmitting electrodes 12 a, 12 b. Inaddition, the number of counter electrodes 14 a, 14 b that defines themaximum measuring length is considerably higher than the number oftransmitting electrodes 12 a, 12 b.

[0045] The signals transmitted by the transmitting electrodes 12 a, 12 bto the counter electrodes 14 a, 14 b are returned via capacitivefeedback to the receiving electrodes 10 a, 10 b. As a result, movingcontacts can advantageously be avoided. The example shown in FIG. 4provides for a single receiving electrode 10, which extends between thetransmitting electrode rows 8 a, 8 b, parallel to these and over thecomplete length. The counter electrodes 14 a, 14 b are respectivelyconnected in pairs in crosswise direction or are formed by capacitancesper unit length 14 in the form of two abutting rectangles that aresomewhat offset relative to each other. The layout of the counterelectrodes 14 can be seen in particular in FIG. 2.

[0046] The triggering unit 4 essentially comprises a clock generator 16and a supply unit 17, which generates the triggering signals startingwith the clocking pulses from the clock generator 16. The clocking ratepreferably is in the range between 10 kHz and 300 kHz.

[0047] The triggering of the transmitting electrodes 12 a, 12 b isexplained with the aid of FIGS. 6a to 6 d. FIG. 6a schematically shows apair of transmitting electrodes 12, designed to represent a transmittingelectrode row 8 a or 8 b, as well as a representative counter electrode14. The receiving electrode is not illustrated herein. By comparison,the counter electrode 14 is twice as wide as the transmitting electrodes12. In the first position P1 shown in FIG. 6a, the counter electrode 14overlays the left transmitting electrode 12 completely and, togetherwith this electrode, forms a partial capacitor C1 of a differentialcapacitor 18. The overlay for the right transmitting electrode 12 inFIG. 6a is only about half, thus resulting in the forming of partialcapacitor C2. The capacitance ratio C1/C2 is 2:1. During thedisplacement movement of the counter electrode 14, the partialcapacitance C2 is initially enlarged distance proportional until bothtransmitting electrodes 12 a, 12 b are completely overlaid. Anotherdisplacement results in a reduction of the partial capacitance C1 withthe partial capacitance C2 remaining the same. In the second positionP2, shown in FIG. 6a, the capacitance ratio C1/C2 is approximately 1:2.The supply unit 17 periodically triggers the transmitting electrode pair12.

[0048] If several transmitting electrodes 12 are arranged in a row, asshown for example in FIG. 6b, and the counter electrode 14 is displacedfurther in the direction V, a changeover to the following pair oftransmitting electrodes is advantageous because of the requiredmeasuring accuracy. In that case, the transmitting electrodes 12 in thecenter with references C1 and C2 are triggered. The designation “0”means that these transmitting electrodes receive either no signal or nocharge packets with alternating mathematical sign, so that theassociated charges on the whole balance out. Each time the capacitanceratio C1/C2 moves from the range 2:1 to 1:2 because of a positionchange, a changeover to the following transmitting electrode pairoccurs. It would also be possible to select a range between 0:1 and 1:0and always change over to the next but one electrode pair. However, thefirst example results in shorter changeover times and lower number ofinterpolation errors.

[0049] With a group of six transmitting electrodes 12, the counterelectrode 14 can be as wide as three transmitting electrodes 12.Respectively one and the next but one transmitting electrode 12 form theactive electrode pair, as shown in FIG. 6c on the top with the firsttriggering option. FIG. 6c on the bottom shows the last triggeringoption, for which the second transmitting electrode 12 in displacementdirection V together with an element 14 of counter electrode row 9 formsthe partial capacitor C2 and the last transmitting electrode 12 togetherwith the next to the last element of the counter electrode row 9 formsthe partial capacitor C1. The counter electrode 14 positioned in-betweenand indicated with dash-dot line is not necessary for the measurement.In general, each second element of the counter electrode row 9 a, 9 b istherefore connected to ground or removed. A group of 6 transmittingelectrodes permit a total measuring distance of six electrode widths,approximately 5 mm in practical operations.

[0050] Even larger groups and thus additional measuring distances can beformed in this way, wherein the number of non-triggered transmittingelectrodes 12 that are positioned between the partial capacitors C1 andC2 is correspondingly increased. In addition, several such groups can belined up in a row and transmitting electrodes with a specific ordinalnumber for all the groups can be triggered jointly, as shown for examplein FIG. 6c on the bottom. All simultaneously triggered transmittingelectrodes in that case define the respective partial capacitance C1 orC2 of the differential capacitor 18.

[0051] If 8 or more transmitting electrodes form a group, two or moreside-by-side arranged transmitting electrodes 12 can also be combinedand are triggered jointly (FIG. 6d). There are many options forarranging and activating the transmitting electrodes 12 a, 12 b.

[0052] As previously indicated, the desired capacitance ratio C1/C2 isdetermined with the aid of an interpolation method, for which theprocessing unit 5 is intended. The processing unit 5 includes asigma/delta converter unit 21, a switch unit 22, a counter unit 23 aswell as an evaluation logic or an evaluation unit 24.

[0053] The sigma/delta converter 21, illustrated in FIG. 7, is an analogcircuit that essentially comprises an integrator 26, the input of whichis connected via a switch S1 of switch unit 22 to the receivingelectrode 10. It also comprises a comparator 27, which is connected tothe output of integrator 26. This integrator is formed in the standardmanner by an operational amplifier 28 and an integration capacitor C3,arranged in the feedback path of integrator 26. The input and the outputof integrator 26 are respectively connected via capacitors C4, C5 toground. These capacitors have only low capacitances as compared to theintegration capacitor C3 and function to suppress interference in thecharges caused by the switch unit 22.

[0054] The switch unit 22 comprises two additional switches S2 and S3.The switch S3 is arranged in the feedback line for integrator 26, inseries with the integration capacitor C3. The switch S2 is connectedparallel thereto and permits a short circuit between input and output ofthe integrator 26.

[0055] The output of comparator 27 is connected via a sweep stage 29, inparticular a D flip-flop, to the counter unit 23 and, if necessary, tothe evaluation unit 24. The counter unit 23 is provided with twocounters for each capacitive sensor 7 a, 7 b, which count the number ofcurrent surges or charge packets n1, n2 that are transmitted andintegrated via the individual partial capacitors C1, C2. The counterunit also contains a counter z that indicates the detected paircombination of transmitting electrodes 12 a, 12 b within the group. Thecounters n1, n2 determine the resolution, which can be selected to bealmost optionally large. In general, a resolution between 256 (8 bit)and 4096 (12 bit) is suitable for real-time use with low processing timeand high accuracy.

[0056] The mode of operation of the distance measuring system describedso far is described with the aid of FIG. 8, in connection with FIG. 13,which illustrates the program sequence for detecting the position. Thedistance measuring system according to the invention functions asfollows:

[0057] Insofar as a relative measuring system is used, this system mustbe moved to the starting position prior to the measuring operation toobtain defined starting conditions. The distance measuring system 1 isswitched on and the counters n1, n2 and z are initialized. Followingthis, the material measure 3 can be moved to the position to bedetermined, relative to the sensor head 2. With the embodiment asabsolute distance measuring system, the material measure 3 can be movedbefore the distance measuring system 1 is switched on and the countersn1, n2 and z are initialized.

[0058] Following this, a first pair of transmitting electrodes 12 a aretriggered in row 9 a (block 41 in FIG. 13), which pair forms thecorresponding partial capacitors C1, C2 of the differential capacitor18. FIG. 8 in particular shows that the partial capacitors C1, C2 aretriggered with periodic, positive rectangular pulses, wherein thetriggering signals of the partial capacitor C2 (C2 signals) follow thosefrom the partial capacitor C1 (C1 signals) with a phase offset of 90°.The triggering signals generate current surges or charge packets withdifferent polarity on the receiving side, which essentially coincidewith the edge changes and are allowed to pass to the integrator, wherethey are integrated once the switches S1 and S3 are closed. FIG. 8 showsthat the switches S1 and S3 are closed, if possible, during thefollowing cycle, for example during the rising edge of the C1 signal, ifthe comparator output, meaning the total charge amount integrated in theintegration capacitor C3, is negative or during the descending edge ofthe C2 signal if the total charge, meaning the comparator output, ispositive (block 42 in FIG. 13). Thus, a corresponding positive ornegative charge packet is received and added up (blocks 43 a, b in FIG.13).

[0059] The switches S1, S3 remain closed only for a predeterminedduration T to define time windows 33 (FIG. 8) in which selectedtriggering signal edges are positioned. The phase offset, the timewindow 33 and its duration T are fixed, such that only one selectedsignal edge is positioned inside one time window 33 and that the timewindow 33 essentially covers only the edge change. The smaller the timewindow 33 and the better it is synchronized with the signal edges of thetrigger signals, the higher the influence of static interference and thenon-linearity resulting from interference signals. Higher frequencyinterference, for example electrostatic discharges introduced from theoutside via the material measure 3, or charges that move in thedielectric of the scale cover owing to mechanical vibrations, can besuppressed effectively. Determining the time window 33 is thereforefundamentally important to the measuring accuracy or even for thefunctionality of the distance measuring system.

[0060] The number of evaluated C1 or C2 signals (blocks 43 a, b in FIG.13) is counted with the counters n1 and n2. Before the next signal istransmitted, the switches S1 and S2 can be closed and S3 can be opened.Since the integrator 26 input is connected to virtual ground, a chargecurrent can discharge and the partial capacitors C1 and C2 can bepolarized with the opposite polarization without this changing thecharge of integration capacitor C3. This measure improves the linearityof the distance measuring system 1.

[0061] If the triggered transmitting electrode pair 12 a is positionedoutside of the preferred range of, for example, 1:2 to 2:1, therespective following pair is triggered. The necessity of changing thetransmitting electrode pair is detected in that charge packets from thesame partial capacitor C1 or C2 are integrated several timessuccessively. FIG. 13 shows that additional counters Z1 and Z2 are usedfor this, which count the number of successively integrated chargepackets (blocks 43 a, b in FIG. 13). If one of the counters Z1 or Z2reaches the value 4 (or 3), a change to the following electrode pairmust take place (blocks 44 a, b in FIG. 13). In that case, the counter zis incremented or decremented, the counters n1 and n2 are reset and theintegration capacitor C3 is emptied (switches S2, S3 are closed);compare the blocks 45 a, b in FIG. 13.

[0062] If one of the counters n1, n2 reaches a value specified by thedesired resolution (blocks 46 a, b in FIG. 13), the value of the othercounter n1 or n2, which does not reach the end value, corresponds to theinterpolation value within the electrode pair. The capacitance ratioC1/C2 in the reference system belonging to the sensor 7 a is thusdetermined. With a relative measuring system, the exact position of thematerial measure 3 can be determined as follows from the counterreadings n1, n2 and z (block 47 in FIG. 13):

position=n 1−n 2+z×resolution.

[0063] Insofar as an absolute measuring system 1 and only a singleprocessing device 5 are provided, the measuring cycle is subsequentlyrepeated with the sensor 7 b and the capacitance ratio C1/C2, meaningthe counters n1, n2 and z, are determined for this as well. If thecapacitance ratios C1/C2 for both measuring systems, meaning theassociated counters n1, n2 and z, have been determined, a linking unit30 determines the absolute position from this. FIG. 12 shows this in asimplified view. To determine the absolute position, the linking unit 30can utilize, for example, the unambiguous connection between thedifference of the capacitance ratios C1/C2 of both relative measuringsystems and the displacement.

[0064] The transit time offsets that interfere with the serialprocessing of measuring values, particularly at high process speeds, canbe avoided if a separate processing unit 5 is provided for each relativemeasuring system. In that case, the conversion can be almostsynchronous. A single sigma/delta converter 21 is advantageouslysufficient if the integrator 26 is provided with an additionalintegration capacitor C3 b that can be added via a switch S3 b, as shownin FIG. 7. During the measuring operation, the integration capacitors C3or C3 b switch back and forth constantly for the synchronous conversion.Prior to each integration, the switches S1 and S2 can be closed and theswitches S3 and S3 b can be opened to discharge a charge that may stillexist in the lines, so that only the charge belonging to the same sensor7 a or 7 b reaches the associated integration capacitor C3 or C3 b.

[0065] Additional and advantageous modifications of the invention areillustrated in FIGS. 5 and 9 to 11. Insofar as the design and functioncoincide with the above-described distance measuring system, the samereference numbers are used and we refer to the above description.

[0066]FIG. 9 shows an embodiment of the processing unit 5, comprising asigma/delta converter 21 and four charge buffers 31 a, 32 a, 31 b, 32 bconnected in series. The charge buffers 31, 32 a are assigned to thepartial capacitors C1 or C2 of the first sensor 7 a and the chargebuffers 31 b, 31 b[sic]¹ are assigned to the second sensor 7 b. Thecharge buffers 31 a, 32 a, 31 b, 32 b have identical designs and aretraditional Sample-and-Hold elements with respectively one operationalamplifier 32 and one associated charge capacitor C6. The chargecapacitors C6 are charged, if possible synchronous, with 1 to 4 or evenup to approximately 16 charge pulses from the associated partialcapacitor C1 or C2. The switches S4 and S5 used for this are essentiallyclosed from shortly before until shortly after the corresponding signaledge to prevent interference signals from being integrated during thecomplete pulse duration. Additional switches S6 serve to discharge thecapacitors C6 as well as polarize the partial capacitors C1 and C2 priorto receiving a charge pulse. A connection to the evaluation unit 24 canbe established via a switch S7 at the output of each charge buffer 31 a,32 a, 31 b, 32 b and a joint coupling capacitor C7. If the chargebuffers 31 a, 32 a, 31 b, 32 b are charged, their output voltage isproportional to the variable for the respective partial capacitor C1 orC2 of an associated sensor. These voltages consequently can be evaluatedlater on and slowly. A good, synchronous position detection with onlyone processing unit 5 is advantageously possible in this way.

[0067] In order to increase the signal-to-noise ratio, the transmittingand measuring signals can be inverted periodically, for example duringeach second cycle. As a result, the energy from low-frequencyinterference, for example stemming from the activation of the displayunit 6 or caused by mechanical vibration of electrical charges in thedielectric for the scale covering, are for the most part canceled in theintegrator 26. An inversion of the triggering signals can be realizedwith the supply unit 17, for example by using an exclusive OR gate. Anedge change can also be realized during the evaluation after each secondclock cycle. An integrator circuit suitable for demodulation is shown inFIG. 10. In this circuit, an additional switch S8 is connected to theconnection point for switch S2 and the integration capacitor C3 and anadditional switch S9 at the connecting point for switch S3 and theintegration capacitor C3. These additional switches are respectivelyconnected to the output of integrator 26. The integration capacitor C3is charged up as before, with the switches S3 and S8 closed. Withinverted transmitting and measuring signals, on the other hand, theswitches S2 and S9 are closed to reverse the integration capacitor C3,meaning for a reversed charging up. The amount of the integrated totalcharge is thus always changed correctly. Of course, the comparatoroutput must also be inverted periodically for this measure.

[0068]FIG. 5 shows one advantageous embodiment of the capacitive sensor7 a, 7 b, wherein the layout of counter electrodes 14 a, 14 b can beidentical to the one shown in FIG. 3. FIG. 5 also shows that a receivingelectrode 10 can be omitted if the transmitting electrodes 12 a and 12 bcan be switched between transmitting and receiving operation. Theelectrodes 12 a are used as receiving electrodes if the transmittingelectrodes 12 b are in the transmitting mode and vice versa. In thisway, the largest possible electrode surfaces can be created on a givensurface of the sensor head 2 for two parallel relative measuringsystems, which results in a good ratio between useful signals andinterference signals, as well as a low number of interpolation errors.The requirements with respect to internal noise in the evaluationelectronics and the susceptibility to interfering outside signals arethus reduced.

[0069] If the transmitting electrode rows 8 a, 8 b consist of 1 toseveral groups of transmitting electrodes, for example 6, with ameasuring distance of approximately 5 mm (compare FIG. 6c) and the cyclelengths of the material measure 3 differ by {fraction (1/64)}, themaximum measuring distance is approximately 320 mm. With a variant ofthe material measure 3, shown in FIG. 11, the two parallel relativemeasuring systems described so far can be expanded by another measuringsystem and the measuring width can be increased almost optionally.

[0070] As shown in principle in FIG. 11, the width of the one row 9 awith counter electrodes 14 a is reduced in sections, crosswise to thedisplacement direction V. The width of the other row 9 b remainsunchanged. In the position according to FIG. 11, the transmittingelectrodes 12 a are overlaid only partially by the counter electrodes 14a. In the section to the left of stage 34, which is visible in FIG. 11,the transmitting electrodes 12 a are overlaid completely. Several suchsections can be provided. In two to four sections, the measuring rangeis expanded to 500 mm or 1 m. The partitions are preferably stored inthe processing unit 5 prior to the initial start-up. In the same way,the capacitance values C1, C2 or their ratio C1/C2 are stored, which areobtained during a complete overlay, for example in the first section, ifall transmitting electrodes 12 a or 12 b are triggered parallel.

[0071] During a measurement, the position described in the above isinitially determined. Subsequently, all transmitting electrodes 12 a or12 b are triggered sequentially and the respective partial capacitancesor their ratios are determined, wherein a lower resolution is sufficientfor this, for example between 4 bit (16) and 8 bit (256). By comparingthe determined values to the stored values, the active section and fromthis the absolute position is determined. Alternatively, it is possibleto use for the third measuring operation only the transmittingelectrodes used for the preceding position measuring with the first twomeasuring systems, which are overlaid over the total width indisplacement direction.

[0072] A distance measuring systems 1 comprises a capacitive sensor 7 a,7 b designed as differential capacitor 18, the partial capacitors C1, C2of which have capacitances that depend on the position to be determined.The system also comprises a processing device 5, for example with asigma/delta demodulator, for determining the searched for distance. Thepartial capacitors C1, C2 are periodically triggered with binarysignals, wherein the trigger signals of the one partial capacitor C1 aretransmitted phase-offset to those of the other partial capacitor C2. Theprocessing device 5 determines which trigger signals must be used forthe evaluation. In time windows synchronized with the trigger signaledges, a switch unit 22 allows the associated receiving signals to passthrough to the processing device 5 and blanks out all other signals. Thedistance measuring system 1 makes it possible to have few interpolationerrors with high resolution and long interpolation periods.

[0073] The distance measuring system 1 can be expanded with twoessentially independent, parallel operating relative measuring systemsto form an absolute distance measuring system, which determines theabsolute position solely from the current measuring values. Addinganother relative measuring system to the distance measuring system 1will expand the measuring range almost optionally.

1. A capacitive distance measuring system (1), in particular forlength-measuring devices, comprising at least one capacitive sensor (7a, 7 b) designed as differential capacitor (18), which includes at leasttwo transmitting electrodes (12 a, 12 b) and one or several counterelectrodes (14 a, 14 b), arranged opposite and at a distance to therespective transmitting electrodes (12 a, 12 b), such that they can bedisplaced relative to these electrodes and at least two partialcapacitors (C1, C2) are formed, wherein the capacitance of at least onepartial capacitor (C1, C2) changes proportional to the distance, furthercomprising a trigger unit (4), designed to feed binary trigger signalswith rising and descending edges at predetermined moments to selectedtransmitting electrodes (12 a, 12 b) that form partial capacitors (C1,C2) of the differential capacitor (18) for generating measuring signals,wherein the trigger signals from the two partial capacitors (C1, C2)have a fixedly determined phase offset relative to each other, alsocomprising a processing device (5) that evaluates the measuring signalsfor determining the partial capacitances (C1, C2) of the differentialcapacitor (18) and from this the distance to be measured, as well ascomprising a switch unit (22) for defining time windows in whichselected edges of the trigger signals are positioned, so that theassociated measuring signals can pass through and be evaluated in theprocessing unit (5).
 2. A distance measuring system according to claim1, characterized in that the trigger signals are essentially rectangularsignals and that the phase offset between the trigger signals is largeenough, so that only one selected signal edge can be observed within atime window.
 3. A distance measuring system according to claim 1 or 2,characterized in that the phase offset essentially is 90°.
 4. A distancemeasuring system according to claim 1, 2 or 3, characterized in that thetrigger signals from the one partial capacitor (C1, C2) have the samecurve shape and the same polarity as the trigger signals from the otherpartial capacitor.
 5. A distance measuring system according to claim 1,characterized in that the processing device (5) contains a sigma/deltaconverter (21), which includes an integrator (26) that integrates thecharge transmitted via the differential capacitor (18), as well as acomparator (27) that issues the mathematical sign for the completeintegrated charge.
 6. A distance measuring system according to claim 5,characterized in that the switch unit (22) permits a measuring signalfrom the one or the other partial capacitor (C1, C2) of the differentialcapacitor (18) to pass through in dependence on the comparator (27)output.
 7. A distance measuring system according to claim 1,characterized in that the time window is smaller than the pulse durationof a trigger signal.
 8. A distance measuring system according to claims5 and 7, characterized in that the time window is dimensionedsufficiently large so as to nearly completely integrate the receivedcharge packets in an integration capacitor (C3) of the integrator (26),taking into consideration the internal resistance of the source feedingthe integration capacitor (C3).
 9. A distance measuring system accordingto claim 8, characterized in that the time window is selected largerthan the sum of the rise time and ten times the time constant (τ), whichis defined as the product of the internal resistance of the voltagesource used and the maximum partial capacitance (C1, C2).
 10. A distancemeasuring system according to claim 1, characterized in that the switchunit (22) creates defined voltage ratios at the partial capacitances(C1, C2) before each trigger signal is transmitted.
 11. A distancemeasuring system according to claim 1, characterized in that the switchunit (22) is provided with switches (S8, S9) for eliminatinglow-frequency interference during the evaluation.
 12. A distancemeasuring system according to claim 11, characterized in that a periodicedge change must occur during the evaluation in order to eliminateinterference.
 13. A distance measuring system according to claim 1,characterized in that several transmitting electrodes (12 a, 12 b) areprovided, which are arranged in at least one row (8 a, 8 b).
 14. Adistance measuring system according to claim 13, characterized in thatthe width of the counter electrodes (14 a, 14 b), as measured in onedisplacement direction (V), corresponds to a whole number multiple ofthe width of the transmitting electrodes (12 a, 12 b) and that a row (9a, 9 b) that is preferably formed with several counter electrodes (14 a,14 b) in displacement direction (V) measures a multiple of the length ofthe transmitting electrode row (8 a, 8 b).
 15. A distance measuringsystem according to claim 1, characterized in that each second counterelectrode (14 a, 14 b) is connected to ground or has been removed.
 16. Adistance measuring system according to claim 1, characterized in that anon-contacting pick-up of the measuring signals occurs.
 17. A distancemeasuring system according to claim 1, characterized in that severalindividual pulses are transmitted to each partial capacitor (C1, C2) forthe measuring and that the resulting charge packets are added up,wherein the partial capacitances (C1, C2) are determined throughinterpolation.
 18. A distance measuring system according to claim 17,characterized in that the processing unit (5) comprises a counter unit(23) with two counters (n1, n2) for adding up the number of integratedcharge packets that are transmitted via the respective partialcapacitors (C1, C2).
 19. A distance measuring system according to claim18, characterized in that starting with the counter readings (n1, n2),the trigger unit (4) determines the pair of transmitting electrodes (12a, 12 b) that must be triggered, so that the ratio of partialcapacitances (C1, C2) relative to each other is within a predeterminedrange, preferably in the range of 1:2 to 2:1.
 20. A distance measuringsystem according to claim 1, characterized in that the transmittingelectrodes (12 a, 12 b) are combined into groups containing a number ofelectrodes, preferably 6 electrodes, which offer a corresponding numberof trigger options.
 21. A distance measuring system according to one ofthe preceding claims, characterized in that the distance measuringsystem (1) is designed as absolute distance measuring system, comprisingat least two capacitive sensors (7 a, 7 b) that are designed asdifferential capacitors (18) and which include transmitting electroderows (8 a, 8 b), each having a constant partition that differs for therows, as well as associated counter electrode rows (9 a, 9 b), whereinthe processing device (5) determines an absolute value for the absolutedistance to be measured from the ratios of the partial capacitances (C1,C2) of the individual differential capacitors (18).
 22. A distancemeasuring system according to claim 21, characterized in that thetransmitting electrode rows (8 a, 8 b) extend in a joint plane andparallel to each other and that the counter electrodes (14 a, 14 b) arearranged on a joint surface of a material measure (3) that is facing thetransmitting electrodes (12 a, 12 b).
 23. A distance measuring systemaccording to claim 21, characterized in that only one receivingelectrode (10) is provided, which is arranged between the transmittingelectrode rows (8 a, 8 b) and extends parallel to these and that theindividual elements of the counter electrodes (14 a, 14 b) areconnected.
 24. A distance measuring system according to claim 21,characterized in that for a two-row arrangement of the transmittingelectrodes, alternate rows (8 a, 8 b) of the transmitting electrodes areconnected as receiving electrodes.
 25. A distance measuring systemaccording to claim 21, characterized in that a separate processing unit(5) is assigned to each differential capacitor (18) for determining therespective relative positions and that a linking unit (30) is providedfor determining the absolute position from the two relative positions.26. A distance measuring system according to claim 21, characterized inthat a joint converter unit (21) is assigned to the two differentialcapacitors (18), wherein the two systems have a separate integrationcapacitor (C3, C3 b) that can be added.
 27. A distance measuring systemaccording to claim 21, characterized in that charge buffers (31 a, 31 b,32 a, 32 b) are provided for the differential capacitors (18), which arecharged synchronously if possible with one or more charge pulses, andthat the output voltages of these buffers are later on seriallyevaluated by a joint processing device (5).
 28. A distance measuringsystem according to claim 21, characterized in that an additionalmeasuring system is provided for expanding the measuring range.
 29. Adistance measuring system according to claim 28, characterized in thatthe width of one row (9 a, 9 b) of counter electrodes (14, 14 a, 14 b)is reduced in sections, crosswise to a displacement direction (V), whilethe width of the other counter electrode row remains the same or isincreased in sections, so that specific capacitance ratios exist in eachsection between the transmitting electrode rows (8 a, 8 b) and thecounter electrode rows (9 a, 9 b).